Hearing aids help to compensate for a wide range of hearing impairments that vary in magnitude from mild to severe based on amounts of amplification required to meet hearing threshold levels at predetermined frequencies. Individual patterns of hearing impairment are plotted in audiograms that record a range of hearing threshold levels over a domain of audible frequencies.
Detailed audiological assessments of hearing performance are made by measuring the hearing threshold levels throughout the range of audible frequencies presented as both pure tones and speech. Other measures include air and bone conduction, reflexes, tympanometry, most comfortable loudness level (MCL), loudness discomfort level (LDL), and real-ear unaided response (REUR)--the acoustical influence of the auditory canal and concha.
Various combinations of these measures are used in conjunction with a variety of prescriptive formulae for selecting hearing aid amplification characteristics. For example, one prescriptive formula, known as the Byrne and Tonisson procedure, calculates amplification characteristics required to present important frequency components of speech with equal loudness. Another prescriptive formula, known as the Berger procedure, calculates amplification characteristics required to restore a fraction close to one-half of the measured hearing loss at each frequency.
The prescriptive formulae specify exact amplification characteristics at predetermined frequencies, and these desired characteristics are often referred to as "target frequency responses". However, the ability to meet these target frequency responses with known hearing aids is limited, and procedures for selecting and adjusting hearing aids to approximate the target frequency responses are much less exacting.
One approach allows physicians and audiologists to choose from a large array of electrical components such as microphones, amplifiers, filters, and receivers, each contributing to a total frequency response of the assembled hearing aid. The large number of available components, peculiarities of each component, and the interaction between components make the choice of a complete set of components very difficult and time consuming. The large number of different components also adds considerable inventory, design, and manufacturing costs.
Another approach provides physicians and audiologists with a matrix of frequency responses from which to choose. The electrical components for achieving the target frequency responses are selected in advance by the hearing aid manufacturers. However, the limited choices for frequency response usually preclude a close match with the target frequency response, and the hearing aid performance is correspondingly reduced. Improperly matched hearing aids can also produce distorted or uncomfortable sounds and can obscure information important to the perception of speech.
The ability to match target frequency responses with known hearing aids is also limited by the performance of filters within the hearing aid circuits. Attempts have been made to combine filters in both series and parallel circuits to more closely match target frequency responses. However, any frequencies that are attenuated by a first filter in a series circuit cannot be fully restored to a higher level of amplification. Filters arranged in parallel (i.e., in separate channels) for processing different portions of the audible spectrum produce individual phase shifts that interfere with recombining the two processed portions of the spectrum.
Some hearing aids are also provided with potentiometers to provide a further adjustment to frequency response after manufacture. The potentiometers are used to control the performance of hearing aid components such as amplifiers and filters. The adjustments are often based on subjective responses of the hearing aid wearer and may produce results that are inappropriate for other sound environments.